net: core: fix Null ptr dereference in UID-based routing

sock_i_uid(struct sock *) is being called without checking
for a valid "struct sock" pointer, resulting in following
Null pointer dereference:

[   17.633071] Unable to handle kernel NULL pointer dereference at virtual address 00000198
[   17.641087] pgd = ffffffc07a554000
[   17.644465] [00000198] *pgd=00000000fa49b003, *pud=00000000fa49b003, *pmd=0000000000000000
[   17.652669] Internal error: Oops: 96000006 [#1] PREEMPT SMP
[   17.658186] Modules linked in:
[   17.661217] CPU: 1 PID: 1333 Comm: S21avahi-daemon Not tainted 4.0.0-rc7+ #2
[   17.668195] Hardware name: ARM Juno development board (r0) (DT)
[   17.674055] task: ffffffc976936800 ti: ffffffc079c0c000 task.ti: ffffffc079c0c000
[   17.681470] PC is at _raw_read_lock_bh+0x34/0x54
[   17.686042] LR is at sock_i_uid+0x24/0x64

Not that we need to check for valid pointer everytime we call
sock_i_uid() but plan is to at least do it when we are doing
this Null pointer check for other arguments.

Change-Id: I04521c1c38179687acd91d155ea2fd4658573ae7
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>